Geochemistry of Venus ’ Surface : Current Limitations as Future Opportunities

Geochemical data about Venus' surface materials are quite limited and of poor precision. The Venera and VEGA lander missions (sources of the available data) were engineering and scientific triumphs, but their chemical analyses of the Venus surface do not permit detailed confident interpretation, such as are routine for terrestrial analyses and MER APXS rover analyses from Mars. In particular, the Venera and VEGA analyses of major elements (by XRF) did not return abundances of Na, and their data on Mg and Al are little more than detections at the 2 level. Their analyses for K, U, and Th (by gamma rays) are imprecise, except for one (Venera 8) with extremely high K contents (~4% K 2 O) and one (Venera 9) with a non-chondritic U/Th abundance ratio. The landers did not return data on other critical trace and minor elements, like Cr and Ni. In addition, the Venera and VEGA landers sampled only materials from the Venus lowlands – they did not target sites in any of the highland areas: shield volcanos, tesserae, nor the unique plateau costruct of Ishtar Terra. These limitations on current understanding of Venus' geochemistry emphasize the huge opportunities in additional chemical analyses of Venus' surface. Currently available instruments could provide much more precise analyses for major and minor elements, even within the engineering constraints of the Venera and VEGA lander systems. Such precise analyses would be welcome for basalts of Venus' lowland plains, but would be especially desirable for the highland tesserae and for Ishtar Terra. The tesserae may well represent ancient crust that predates the most recent volcanic resurfacing event and so provide a geochemical look into Venus' distant past. Ishtar Terra may be composed (at least in part) of granitic rocks like Earth's continental crust, which required abundant water to form. So, Ishtar Terra could possibly yield evidence on whether Venus once had an ocean, and thus the possibility of life. This chapter focuses on the chemical composition of Venus' surface, direct analyses and indirect constraints, emphasizing how little is actually known. This incognizance of Venus' geochemistry highlights the opportunities for high-impact scientific return from future investigations, by remote sensing, in situ analyses, and sample return. At this time, it is fair to say that we know little of Venus' geochemistry. What we do know is greatly limited, both with respect to spatial/geologic coverage and analytical precision. This chapter does not review, or …